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William D. McCaffrey1, Benjamin C. Kneller2, Omar Al-Ja'aidi2
(1) Leeds University, Leeds, United Kingdom
(2) Leeds University

Abstract: Flow efficiency controls on the geometry and location of turbidite sheet systems

Flow efficiency gives a relative measure of the distance a flow may travel. It is a characteristic of the flow, and is therefore independent of topographic effects. In flows of higher efficiency, the transport phase is prolonged, thus elongating the deposit, or displacing it distally. Although early workers stressed the importance of the fine-grained component of the sediment budget on flow efficiency, new work outlined here illustrates that this is only one of a number of linked controls, which include: 1. the suspended sediment grainsize distribution, 2. suspension density and 3. flow volume. The presence of a significant fine-tail to the sediment budget controls flow efficiency through the mechanisms of buoyancy enhancement for the coarser-grained suspended sediment component, reduced rates of momentum loss due to low mud deposition rates and friction reduction; suspension density affects flow efficiency principally through its impact upon the flow momentum, whilst suspension volume controls flow efficiency principally by affecting the shear velocities. We present experimental data on scaled, sediment-bearing turbidity currents which model the deposition of basin-floor turbidite sheet sands. These illustrate that each of the controls outlined above has a characteristically different effect upon the volume and geometry of the resultant deposit. Determining paleo-flow efficiency can be important when predicting the loci of sand deposition, and we discuss the pitfalls of making this assessment from core. Recent work has illustrated that cycles in the calibre of sediment delivered to the shelf edge may be related to sea-level, and we evaluate the effects of corresponding allocyclic cyclicity in flow efficiency upon large-scale system development.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana